Coordinate measuring machines (CMMs) are employed to determine the coordinates, in some specified frame of reference, of points on the surface of a workpiece. CMMs are typically employed for digitizing or imaging, whether for replicating a prototype, for example, or for various manufacturing applications. The salient parts of a CMM include a stage, or complex of stages, for moving the object to be characterized, a probe for measuring the distance to a point on the surface relative to a fiducial position, a control or computing system, and measurement software for converting the measurement into a meaningful format for the intended application.
Various probes may be employed for measuring a distance to the surface of the measured workpiece. Any probe, currently known or otherwise, is within the scope of the present invention.
One form of probe, typified by the ConoProbe™ supplied by Optimet, Optical Metrology Ltd. of Jerusalem, Israel, employs conoscopy, a form of holography. Conoscopy is an interferometric technique capable of determining the distance to one or more points on an object surface without employing a reference beam. Instead, light emanating from a source region is prepared in a defined state of polarization and then passed through an anisotropic optical element in which one polarization suffers phase retardation with respect to the other. The two polarization components emerging from the anisotropic optical element interfere with one another, producing a interferogram in the detector plane. Conoscopy is the subject of various patents, including U.S. Pat. Nos. 4,602,844, 4,976,504, 5,081,540, 5,081,541, and 5,953,137, all of which patents are incorporated herein by reference.
One limitation imposed by existing CMMs is due to the fact that even the most versatile optical sensors are unable to digitize on vertical or very steep angles measured with respect to the optical axis (or ‘line of sight’) of the probe. ‘Vertical’, in this case, refers to the surface of the scanned body lying parallel to the optical axis of the probe. An ‘undercut’ refers to a negative angle relative to the line of sight. Some applications, however, such as dental surface profiling for purposes of reconstruction, orthodontics, etc., as well as digitization of plastic parts, molds, etc., require measurements on vertical walls or low angle undercuts.
As used herein, a body characterized as ‘complex’ is one having vertical walls or low angle undercuts. Using prior art technology, scanning a complex body requires orthogonal scanning the object about multiple (typically 5) axes. As used herein, ‘orthogonal scanning’ refers to scanning of the line of sight of a probe entirely within a single plane normal to an axis of rotation. This method, while algorithmically simple, requires very large travel on the scanning stages making the equipment very expensive.
Another prior art solution to the problem of small (or zero, or negative) angles with respect to the probe line of sight is to perform non orthogonal scanning by using a 2-axis angular arm. In this case the whole sensor is rotated, and both the complex arm and the requisite large travel ranges add to the cost of such systems. Yet another prior art solution for scanning complex bodies requires changing the sample position to allow direct line of sight for each feature. In this case very complex reconstruction software is required to merge the individual scans by ‘best fit’ of complex surfaces. The results of the ‘stitch’ depend on the quality of the data, size and shape of common features used for reference and the robustness of the algorithms. Typically, operator intervention is required, both during the scanning (otherwise 2 or more motorized axis are required on the sample fixture) and during data processing. In some cases like dental applications or plastic parts with smooth surface features it is very difficult to find the right fit and reference items to register successive ‘views’ of the object. One solution is to ‘glue’ registration features (‘balls’) to the sample.
Clearly, an automated and robust solution to the problem of digitizing complex bodies is desirable.